1. Field of the Invention
The present application relates to a method to control a tunable laser diode (hereafter denoted as LD).
2. Related Background Art
An apparatus to control an emission wavelength of a tunable LD has been known, where the apparatus includes a thermo-electric cooler (hereafter denoted as TEC) that mounts a tunable LD and a temperature sensor such as a thermister thereon. The apparatus performs a sequence to sense a temperature of the tunable LD, and to feedback this sensed temperature to the TEC to adjust the temperature of the tunable LD. The wavelength division multiplexing (hereafter denoted as WDM) system under developing requires precise control of the emission wavelength of an LD as an optical signal source.
In another aspect of the optical communication system, a tunable LD whose emission wavelength is precisely adjustable to a target wavelength, which is one of the grid wavelengths of the WDM system, has been attracted. Conventional tunable LDs generally configure with a gain region having a gain spectrum with a plurality of peaks and a wavelength selective region generally having a reflection spectrum with a plurality of reflection peaks. Such a tunable LD may emit light with an emission wavelength at which one of gain peaks attributed to the gain region coincides with one of reflection peaks attributed to the wavelength selective region. Tuning the gain region, for instance, by varying a current injected thereinto and the wavelength selective region, for instance, by varying a local temperature thereof, and varying a temperature of the tunable LD, the tunable LD may emit light with the target wavelength.
Such a tuning of the emission wavelength generally requires a time until the emission wavelength becomes stable at the target wavelength. One standard set in the optical source for the WDM system requires the switching of the target wavelength, that is, from receiving the command to switch the target wavelength to stabilizing the wavelength at the target one to be several tenses of milli-seconds at most. The TEC generally dominates the switching of the emission wavelength, which sometimes requires several seconds.